In the field of robotics there has been considerable development of robotic arms having a tip following capability; that is to say, the arm is capable of advancing longitudinally of itself in a snakelike manner. Such arms can carry a workload or tool and can be used for inspection of equipment with restricted access, such as the internal parts of a jet engine.
Tip-following devices in general are used extensively in the medical field in the form of endoscopes and to some extent in industry as boroscopes. These devices, however, rely upon the external environment to guide the work head or inspection mechanism to an appropriate location at which to perform a given task. Endoscopes are usually guided by body orifices and while a certain amount of control may be exercised at an extremity of an endoscope, the fact remains that the main part of an endoscope is essentially passive in which the only guidance is provided by the orifice or conduit within which the device moves.
A major advance in tip following technology for robotic arms is described in our co-pending patent application No. WO 0216995. This application describes and claims a robotic arm comprising a plurality of longitudinal segments, each of which comprises a plurality of passive links. The end of each segment is “guided” by wires so that by varying the length of the wires, the arm can be caused to bend. In a typical arm, it is possible to have a number of segments, say 4 to 20, each containing say 10 to 20 links. By adjusting the tension in the control wires for each segment, the arm can move and adopt various spatial shapes and configurations. This may be done for example by winding each control wire on or off a spindle using a motor. The motors are controlled for example by a computer control system.
The arm is mounted on a base and advanced towards the target, whilst the shape is continuously altered to “follow” the tip.
With such an arm, the articulation of the links is maintained under compression, i.e. by providing tension in each of the control wires, so as to “stiffen” each articulation. This gives the arm spatial determinacy, and results in greater control over the spatial positioning of the arm. Thus it is possible to control a group of links within a segment rather than each link individually using the stiffness of the link design to transmit loads so that any movement and/or load change or moment change is distributed among the links in the segment. Our co-pending application number WO02/100608 also relates to robotic arms where, by interposing a layer of rubber or elastomer either bonded or keyed to two members constituting the articulation between adjacent links within a segment, the rubber layer constitutes a fixed contact surface between the articulated components while at the same time providing the resilient shear capacity necessary to produce “stiffness” of the joint.
In order to provide an arm which is capable of tip following along a predefined path in space in which there is little room for variance or deviation from the defined path, it is necessary to know the exact position and configuration of a large number of elements or components of the arm one with respect to the other. In an “ideal” segment with no friction, and in the absence of externally applied forces such as payloads, gravity, friction etc, each segment will adopt the shape of an arc of a circle having a substantially constant radius of curvature along its length. Furthermore, measurement of the angular displacement of the wire drum or capstan would give an accurate measure of control wire length.
In reality, however, external factors have an effect on each segment, which can alter the shape such that it is no longer a true arc of a circle. Also the wire may slip on the drum under changes in the load on the arm. This makes it more difficult to control the arm to follow a predetermined path, since changes in the attitude of the arm will change the load, weight/friction distribution and hence the shape of the arm itself, causing it to deviate at least in part from the desired path. While some deviation from the desired path may be acceptable in some situations, other deviations certainly will not be.